The present invention relates to a device and to a method for applying a medium in liquid, powder or paste form to a substrate, to a system having a plurality of such devices, as well as to a use of such device, method and system.
The substrate in question is preferably a textile substrate, although large area substrates may also quite generally be used, for example a substrate made of foil, nonwoven fabric, metal, carpet, plastic, paper, wallpaper, wood, glass, porcelain, ceramic or a similar material. The substrate may also be a printing support, for example a printing plate or a printing roll, to which it is necessary to apply printing ink as a medium prior to printing on a substrate made of paper, wallpaper etc. The advantage is that the medium can be applied at specific points on the printing support. With the medium a pattern is to be applied to such a substrate with the sharpest possible contours and a high resolution.
Corresponding to the large number of different substrates indicated above, many methods and devices for patterning surfaces of such substrates are also known. If they are to function with a printing speed which is high enough for mass production, these methods and devices basically require stencils which are per se expensive to produce. An example which may be mentioned in relation to this is that of the textile printing industry: for the screen printing preferably employed in this case, millions of printing stencils are made year after year just for rotary and flatbed printing. A large number of gravure cylinders are also produced for printing on films.
For rotary screen printing, a distinction is essentially made between two ways of producing the stencils: these are directly patterned DEP stencils (DEP=Direct Electrolytic Patterning) and resined stencils. The DEP stencils have the pattern electrolytically applied directly to them and can thus be used without further etching. With DEP stencils, the pattern and the colour separation are therefore already incorporated into the relevant dies.
Conversely, for the production of resin stencils, cylindrical screens are firstly produced electrolytically in a relatively complicated way. Various etching resists are then applied, according to the etching technique which is being used. All the openings existing in the stencils are closed with the etching resist during this. The desired printing pattern is then created by controlled release of openings for the respective colour separated beforehand from the model. This procedure can be carried out either using photographic development and wet chemical washing of the resist, or by direct digital transfer of the information using a laser device which xe2x80x9cburns offxe2x80x9d the etching resist using a laser beam.
Resin stencils have the advantage over DEP stencils that, by removing and re-applying the resist, they can be reused many times for different designs, whereas the DEP stencils can be used only for one design.
In short, the production of printing stencils, whether rolls or plates for typographic or gravure printing on paper and film, or screen stencils for rotary printing or flatbed printing on textile or carpet, is elaborate. The same is also true as regards the resin stencils which can be used repeatedly, since with these, for repeated use, the resist firstly needs to be removed, after which resist is again applied, dried etc. and this is followed by etching.
The production of such stencils finally only leads to economically viable products, for example printed textile, if the stencils are produced in large numbers and can be employed over an extended period of time for printing on large numbers of articles.
Now, especially in textile printing the problem arises that the time for which printing patterns are regarded as fashionable is becoming ever shorter, and at the same time the variety of patterns is increasing continually. Always producing new stencils therefore leads, every time fashion changes, to new rises in costs for every shorter xe2x80x9cyardagesxe2x80x9d. This means that, especially in printing businesses in Europe and the USA, there are commonly large stocks of out-of-date stencils, the number of which may amount to several tens of thousands of stencils.
It should also be pointed out that stencil production as a whole, as well as stencil recycling, are very environmentally unfriendly and involve a large consumption of energy.
In view of this situation, consideration has already been given, for printing on textiles, to abandon the screen printing method and, for example, employ a digital inkjet printing method, successfully used in the paper industry, in order to transfer a pattern to textiles. In a method described in U.S. Pat. No. 4,324,117, liquid droplets are sprayed from very fine nozzles onto well-defined points on a substrate. The color mixing is in this case carried out with up to eight colors per point. Each of the eight colors can be applied in 256 levels. In spite of this variety of colors, the color space which can be obtained is limited in comparison with the color space of the screen printing method.
Thus, inkjet printing methods do indeed have the advantage that it is possible to avoid the elaborate production of stencils, that they furthermore make it possible to print without regard to register, and that it is unnecessary to premix color pastes. However, industrially usable production systems which make it possible to produce large yardages have not yet successfully been made. Individual systems have to date operated in the field of patterning with a printing speed of at most 1 m/min., while the average printing speed of a rotary printing machine is about 40 to 120 m/min.
It should moreover be taken into account that, with the inkjet printing methods, the droplets are formed within very fine nozzles having diameters in the micrometer range, for example 10 xcexcm. These fine nozzles therefore unavoidably give rise to the problem of their clogging. With such nozzles, it is therefore only possible to use particular categories of color in highly pure form for printing, in order to minimize the risk of the nozzles clogging. The color space is accordingly also limited, and the use of, for example, metallic colors which are needed in fashion to obtain an iridescent effect, is out of the question.
A replacement for screen printing with stencils, which is suitable for mass production, has thus not yet been successfully found.
From the abundant prior art, only a few documents will be dealt with below by way of example:
DE 31 37 794 C2 describes a device for continuously delivering a minimal amount of liquid to a web of material. This device has a fine-meshed screen and a blowing device directed against the screen. The screen rests in this case as a textile mesh belt without pressure on the web of material, or is guided or laid over it, and the blowing device is arranged above the mesh belt section carrying the ink.
As a supplement to this, DE 31 46 828 C2 proposes using a bath as a liquid delivery device, and arranging the blowing device behind and at a higher level than the delivery device in the running direction of the endless screen belt. Such a device could per se be used for patterning/printing if etching is carried out beforehand.
DE 40 01 452 A1 describes a device for continuously delivering a liquid to a web of material, having a moving screen, means for filling the openings in the screen and a blowing device for transferring the liquid held in the openings in the screen onto the web of material. The device for filling the openings in the screen consists of chambers which are arranged opposite one another on both sides of the screen and bear on the screen, one chamber being designed as a feed chamber and being connected to a liquid feed, while the other chamber is designed as a discharge chamber and is connected to a liquid drain.
Furthermore, DE 42 28 177 A1 discloses a device for continuously delivering a liquid to a web of material having a moving screen, having filling chambers which are arranged opposite one another on both sides of the screen and extend over the width of the screen, squeegees engaging the screen on both sides and a blowing device which is made of an elongate nozzle which extends over the width of the screen and is cooperating with a propellant feedline. In order for it to be possible to adjust the delivery of liquid continuously to the width of the web of material, each filling chamber has a piston which is guided in leaktight fashion against the screen and can be moved continuously starting from one end of the filling chamber, the elongate nozzle cooperating with a closure belt which can be moved continuously from one end of the elongate nozzle and allows the elongate nozzle to be closed off to a greater or lesser extent.
Lastly, AT-PS 175 956 furthermore discloses a method and a device for applying liquid materials to a base. The nozzles, which are arranged behind a screen, can likewise be adjusted individually in order to control the respective amounts delivered. This method and this device are used, however, not to pattern the base but instead to coat it so as to provide it with uniform delivery. Through the arrangement of a covering mask, it is possible to adjust the distribution of a medium, taken from a container, on the substrate in a fixed ratio. The use of such masks is, however, not comparable with patterning which can be achieved by printing. Furthermore, no consideration is given here either to the synchronization between the delivery and base which is per se necessary and suitable for patterning.
The object of the invention is to provide a device and a method for applying a medium in liquid, powder or paste form to a substrate for forming a pattern on the substrate, it being possible for a pattern to be applied without the use of etched stencils with, in comparison to inkjet methods, considerably increased printing speed irrespective of registers, customary color categories being usable and the purity of the colors requiring no particular precautions.
A further object is to provide a system which makes it possible to use a plurality of such devices.
The invention is advantageously used for applying a pattern to large area substrates, in particular textile goods, for applying printing ink to particular regions of a printing support and for applying media for patterning supports for printing, especially screen printing, made of metal or plastic.
The device according to the invention and the method according to the invention firstly propose, as a complete departure from the prior art, a separation between the propellant for propelling the medium, that is to say preferably a printing substance, to the substrate and the medium itself. In order to achieve this separation, the liquids used for the medium, for example solutions, dispersions, suspensions etc., or pastes and powders, are distributed in a transport device, preferably in discrete form. In the case of media in liquid or paste form, a capillary action due to small openings in the transport device is employed for filling this device. Specifically, this brings about spontaneous xe2x80x9cfillingxe2x80x9d of the small openings, which leads to virtually xe2x80x9cautomaticxe2x80x9d metering. After apportioning (if necessary) of the medium which is optionally carried out using squeegees, it is transported into the desired delivery zone and delivered from there by the propelling device which is adapted for propelling medium from selectable points of the delivery zone, the propelling device being controlled for selecting said points. The propellant preferably is a fluid, i.e. a liquid or a gas, in particular air. In the case of using a gas as the propellant, a pressure range of between 103 and 106 Pa (0.01 and 10 bar) is used.
Delivering the medium from selectable points of the delivery zone can be used directly (hereinafter: xe2x80x9cdirect methodxe2x80x9d) or indirectly (hereinafter: xe2x80x9cindirect methodxe2x80x9d) for forming a pattern on the substrate, although the direct method and the indirect method share the same inventive idea, and should be regarded as mutually xe2x80x9cinvertedxe2x80x9d printing methods.
In the direct method, the medium propelled from the delivery zone is transferred directly to the substrate, and forms a part of the desired pattern on the substrate. In the indirect method, the medium propelled from the first delivery zone is not transferred to the substrate, but merely removed from the transport device. The medium remaining in the transport device is transferred to the substrate with a delivery device which may be e.g. in the form of a conventional blade squeegee or roller squeegee device (in which the transport device is in contact with the substrate), or may alternatively be a non-selective propelling device (in which the transport device is not in contact with the substrate), e.g. of the type disclosed in AT-PS 175 956. Consequently, in the direct method and the corresponding device, the propelling device selectively propels the medium which is to be transferred from the transport device to the substrate, while in the indirect method and the corresponding device, the propelling device selectively propels the medium which is not to be transferred from the transport device to the substrate.
In a preferred embodiment of the method and device according to the invention, as a propellant short gas pulses are used, which can be selectively released from nozzles connected with controllable valves, thereby selectively releasing amounts of medium from the transport device, in the direct method onto the substrate over its width and length, and in the indirect method into a collecting device, preferably for recycling. The patterning is thus carried out by separating the medium or printing substance from the propellant. Namely, whereas in the case of the existing nozzle devices and methods for delivery to a substrate, a pre-pressurized liquid is used and is converted into droplets by thermal expansion or alternating piezoelectric voltages, a procedure of this kind is superfluous, and moreover unusable, in the device according to the invention and in the method according to the invention. The propellant is blown in the form of gas, preferably air, onto the medium, so that the medium is transferred onto the substrate in the desired way (direct method), or removed from the transport device whereby the remaining medium is delivered to the substrate in the desired way (indirect method).
Any problems in terms of cleaning the nozzles and their clogging are eliminated in the device according to the invention and in the method according to the invention, since in this case the nozzles merely output gas pulses and do not spray the medium.
In the device according to the invention and the method according to the invention, the information which the pattern contains for the respective colors can be obtained from a computer which actuates the nozzles accordingly, so that they deliver the gas pulses in correspondence with the desired pattern.
As is known, the resolution of screen printing is a decisive parameter for its quality. In all screen printing methods and devices, the resolution (that is to say the density of the individual printing points) is rigidly dictated by the resolution of the stencil. This is due to the fact that screen printing methods and devices work exclusively using contact with the substrate, and the velocity between the substrate and the stencil always has, apart from small frictional effects, the same value.
In this regard as well, the present invention in the direct method provides considerable advantages through a resolution that can be varied in a wide range. This variable resolution is actually achieved by separation of the propellant for applying the medium to the substrate from the medium, or printing substance, itself and furthermore by the possibility of adjusting a relative velocity between the transport device, or delivery device, on the one hand, and the substrate, on the other hand, and by the possibility of matching the resolution by appropriately increasing the frequency with which the propellant is sent from the delivery device to the transport device, in order to supply the medium from the latter to the substrate without contact between the transport device and the substrate.
Through selective actuation of individual nozzles in pattern-related synchronization with the substrate, it is possible to transfer arbitrary patterns using the device according to the invention and the (direct) method according to the invention, while also increasing the speed of the transport device so as thereby to achieve variable resolution on the substrate. For example, by doubling the rotational speed of a transport drum which forms the transport device relative to the substrate, and doubling the delivery frequency of the gas pulses, a two-fold increase in the point density on the substrate can be achieved. It is therefore possible to transfer large amounts of colors onto the substrate, which is a great advantage especially in the textile printing industry.
One possible way of patterning a substrate in the direct method consists in moving the substrate to be printed past the device according to the invention, or a system containing a plurality of such devices. The delivery is then carried out selectively over the width of the substrate, and its length, in order to transfer the desired pattern to the substrate without contact.
It is, however, also possible to move the device or the system past a fixed substrate to be printed, or appropriately scan this substrate, while delivering the pattern in the desired way to appropriate regions of the substrate.
In the indirect method the invention provides considerable advantages through the selective delivery of medium in a pattern related synchronization with the substrate, with which is not only possible to transfer an arbitrary pattern, but also to transfer relatively high amounts of medium suitable for textile printing. Substrates can be patterned with high speed, with well known media, and without register. The medium remaining in the transport device, such as a drum or belt, after removal of medium in the propelling device, can be transferred to the substrate by a non-selective propelling device, or e.g. by a well known squeegee operation.
Very precise patterns can be transferred to a substrate using the device according to the invention and the method according to the invention. Care merely needs to be taken in this case that the transport device, e.g. the transport drum provided with holes, or the screen or else a mesh belt guided by rollers, is produced with high accuracy and runs true. This can be readily ensured by electrolytic production of the screen and by a suitable drive mechanism, so that the required balanced running accuracy and synchronization between the transport device and the substrate is achieved.
A further advantage of the invention (direct method) is that the medium is delivered without contact and adhesive bonding, for example of a web of textile goods onto a back cloth in the conventional sense can per se in principle be omitted, although it does not have to be omitted.
The aforementioned synchronization of the transport device with the motion of the substrate can, for example, be achieved by establishing the position of the transport device using an encoder and employing the signal supplied by the encoder to synchronize the gas pulses with the position of the transport device. It is, however, also possible to measure the position of (possibly encoded) individual holes of the transport device, in particular a transport drum, during operation and match the actuation of the valves which supply the gas pulses appropriately to the desired pattern. Electromagnetic acquisition has proved particularly advantageous for ascertaining the position of the transport device. Optical or capacitive acquisition is, however, also possible.
In the above the propelling device has been described as a device delivering gas pulses for conveying medium from a transport device. It is, however, also possible to provide the propelling device with one or more heating devices, e.g. a laser device or a high frequency device, for producing a thermal delivery of amounts of medium from the transport device. One or more of the heating devices can be arranged over the width of the transport device. Laser radiation with d- suitable wavelength can be directed with optics such that separate amounts of medium are released in an explosion-like way due to a very rapid heating, which will be particularly advantageous in the indirect method according to the present invention. A similar effect can be obtained by directional high frequency heating. Another way of delivering amounts of medium is electrostatically.
In the screen printing methods currently carried out, a substrate already prepared for printing, or an article, is customarily subjected to the printing process. In the case of processing natural fibers as a textile substrate, this means that the article has been scoured, decocted and bleached and has a degree of whiteness deemed suitable for printing. These articles which are white enough for printing are then fed dry to the printing process.
Of course, printing on textile goods as a substrate should be guaranteed to have the sharpest possible contours. However, the use of a medium or printing substance with low viscosity, such as the viscosity of a conventional textile printing paste, may be required for technical reasons. Unfortunately, this low viscosity may lead to a lower quality in the resolution and edge sharpness of the printing. Also, the color retention capacity in the case of printing on fabrics and knitwear is poor. Furthermore, because of the existence of a texture, there is a tendency for the printing substance which has been delivered, if it is fluid, to spread or run. In this case as well, it is difficult to form a sharp printing pattern. Optimization should be carried out in this case, but without causing degradation of the other properties of the textile substrate. This is achieved according to the invention through the possibility of having some of the chemicals needed to improve the process delivered to the substrate prior to the actual printing process. These chemicals, also referred to as printing aids, may with this procedure be delivered to the dry substrate, which in other regards has already been prepared for printing, for example with a foulard (bath) or another suitable delivery unit. In certain cases, the delivery of wet printing aids to a wet substrate may also be envisaged. After this, the substrate is then dried to an acceptable residual moisture content of, for example, from 2 to 15% in order for the actual printing process then to be carried out. This entire process step may be carried out both in stages and continuously in one working step.
In printing processes generally premixed colors are used. However, according to the invention it is also conceivable to use primary colors, and to mix these directly on the substrate. This method is already known as xe2x80x9cmultichromyxe2x80x9d, in particular xe2x80x9cquadrochromyxe2x80x9d or xe2x80x9coctochromyxe2x80x9d, and is used in inkjet printing and flat bed printing. The advantage of this method is that metering and mixing of colors can be omitted completely. This is a considerable environmental advantage, since extensive cleaning of buckets and other containers are unnecessary. The number of necessary stencils is reduced, which reduces costs.
The substrate may be a printing support, e.g. a printing roll or printing plate. With the invention, it is possible to feed ink to this printing support in a simple 30 way, with a high degree of control, only over a desired, rather than the entire, width or area. The substrate may comprise metal, plastic, rubber etc.
The substrate may also be a printing form, e.g. a printing screen, for forming a screen printing stencil which is to be provided with a patterning medium. This medium can be a patterning lacquer, a patterning resist, a wax or an ink.
It is to be observed that the method according to the invention can also be used for producing a conventional patterned printing form, in particular a patterned printing screen or stencil, by providing the printing form with a pattern of lacquer or resist. In the indirect method according to the invention, the transport device is a screen, and the medium is the lacquer. The propelling device is used to remove lacquer from selected holes of the screen, which holes are to be used to let pass a printing substance during the use of the printing screen thus obtained.
The claims and advantages will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.